CENTER OF MASS AND MOMENT OF INERTIA 141 or similarly and 2m.' Sm (10 Evidently x, y, and z are the coordinates of the center of mass. ILLUSTRATIVE EXAMPLES. 1. Find the center of mass of two particles of masses m and nm, which are separated by a distance a. Taking the origin of the axes at the particle which has the mass m, Fig. 72, and taking as the x-axis the line which joins the two particles we get - _. Q + nma m + nm n FIG. 72. 2. Find the center of mass of three particles of masses m, 2 m, and 3 m, which are at the vertices of an equilateral triangle of sides a. Choosing the axes as shown in Fig. 73 we have - _ 0 + 2 ma + 3 ma cos 60° m + 2m + 3m = x72 a, -- 0 + 0 + 3 ma sin 60° 6m 2=0.